1. Field of the Invention
The present invention relates to electrical current switching devices which employ or which are actuated in a vacuum environment and more particularly the invention relates to contact materials which are utilized on the surface of the electrodes which are employed in switching electrical current.
2. Description of the Prior Art
In electrical current switching devices, the electrical circuit is opened and closed by making or breaking contact between two electrode structures which are operating within a vacuum environment. Consequently, the contact surface of the electrode structures becomes of critical importance.
In such electrical current switching devices, heat is generated due to the passage of electrical current at locations where resistance is encountered. This occurs at places where the electrodes touch and more severely during opening and closing operations provided that the electrical potential is sufficiently high to cause the flow of current in the form of an electrical arc. One consequence of the heat which is evolved during such operations is the formation of hot spots on the surface of the electrodes. These hot spots can lead to the softening or melting of some of the material and if the electrodes are in contact or brought into contact with one another at such time, especially where there is molten or near molten material on the surface of said electrodes, undesirable welding of the electrodes can occur. It has been found that when such electrodes are employed in a vacuum environment, the problem of welding of the electrode contact surfaces together becomes more accentuated.
A proposed solution to this problem is to recognize that such welding does take place. The characteristics of the weldment and particularly the strength thereof must be adjusted so that the welding which may take place will be of sufficiently low strength that the weld may be readily broken without unduly distorting or changing the surface of the contact material at which said weld occurs. In addition, the fundamental characteristics of the contact materials, namely, good current interruption ability, high voltage withstand capability and low electrical resistance including low chopping and low erosion characteristics must not be altered during operation. In the past various compromises have been proposed with the materials which form the contact surface of the electrodes.
One approach has been to utilize a major proportion of a very strong element and form a sintered network of powdered particles of this material and thereafter infiltrate the same with a lesser amount of another component which will produce a compromise in the various characteristics of the individual components. Typical of such materials has been the employment of a major constituent comprising a refractory metal such as tungsten or molybdenum which is characterized by an exceedingly high melting point and thereby minimize the welding tendency of the electrode. A pure sintered refractory metal contact formed for example, of tungsten, will not provide the required electrical conductivity nor the chopping characteristics and high voltage withstand capability. These characteristics are supplied by infiltrating the sintered matrix with a material of good electrical conductivity and low chopping characteristics but which may suffer from high erosion and lower voltage withstand capability. Notably, such an element is copper or silver. This latter component has always been present in a minor proportion.
Other approaches to the solution of the same problem has involved other compromises and the materials have comprised typically a copper matrix in which copper is the major constituent to which is added another component of limited solid solubility such that between the two components there is a brittle material which will aid in breaking any welds which do occur during arcing when the electrical current switching device is in operation. Typical of these materials are the copper-bismuth and silver-tellurium type contacts. Representative of such prior used contact materials are those described, for example, in the Lafferty et al. U.S. Pat. No. 3,246,979, U.S. Pat. No. 2,246,328 to Smith which discloses the introduction of traces of bismuth in copper for improving the current voltage characteristics of asymmetrical conductors and U.S. Pat. No. 1,375,454 to Hanson which describes an electrical resistance alloy having a chromium content ranging from 30 to 60% together with 30 to 60% of copper and up to 5% of tungsten and molybdenum. Clearly such latter materials while being effective for an electrical resistance alloy are totally unsuited for contact materials for use in a vacuum interrupter.